The present invention relates generally to automatic loop (“autoloop”) system for an electrical power distribution network and more particularly to an autoloop system that eliminates momentary power interruptions on the non-faulted portion of the electrical loop and which prevents reclosers from closing in on known faults.
Electrical utilities have a number of metrics that are used to track performance and customer satisfaction. These metrics, which include the system average interruption frequency index (“SAIFI”), the customer average interruption duration index (“CAIDI”), and for some utilities, the momentary average interruption frequency index (“MAIFI”). SAIFI measures the average number of interruptions that a customer would experience during a time period, such as a year. CAIDI measures the duration of the interruption that a customer would experience, and is generally a few hours per year. MAIFI measures the number of power interruptions that have a duration of less than five minutes that a customer would experience during a given time period.
Some or all of these metrics are also used by government regulators to aid in determining if the electrical utility is adhering to the regulations in maintaining a durable and reliable electrical distribution network. As a result, electrical utility and distribution companies have developed system architectures that minimize the duration and frequency of power outages. One technique uses a device called an “autorecloser” or simply “recloser”, in the protection scheme for the distribution system. A recloser is a type of circuit breaker that includes a mechanism that allows the circuit breaker to close or reconnect the electric circuit. The premise of the recloser is that many electrical faults on the overhead open wire system are transitory, due to issues such as a tree limb touching or falling on a power line for example. The recloser includes a controller that is programmed to make several attempts to reconnect before locking open the recloser. Generally, the feeder recloser is programmed with an operating profile that provides for one “fast” trip and two “slow” trips before finally locking open.
Another technique used to improve performance is to arrange the distribution network into two or more branch circuits that can be adaptively coupled in the event of a failure. Generally, a substation will provide two or more feeder branch circuits to a typical autoloop. A feeder recloser is positioned at the beginning of the circuit and provides the protection functionality discussed above. A second midpoint recloser is positioned at an intermediate position along the branch circuit. Positioned at the end of the branch circuit is a tie recloser that couples the branch circuit with a similarly configured adjacent branch circuit.
The tie recloser is normally in an open position to prevent the flow of electrical current between the branch circuits. In the event of a fault between the feeder and midpoint that the feeder recloser cannot clear, the feeder recloser locks open and results in a loss of potential on the branch circuit. When this occurs, the midpoint recloser changes settings to trip at a lower current level and lock open after only one trip. The tie recloser is arranged to automatically close at short period of time, typically 15 seconds, upon the detection of the loss of potential. When the tie recloser connects the two branch circuits, electrical current can flow into the branch circuit with the electrical fault. Depending on where the fault is located on the branch circuit, partial electrical service can be restored to the portion of the branch circuit that experienced the failure. For example, if the fault occurs between the feeder recloser and the midpoint recloser, when the tie recloser connects the circuits, electrical power flows into the branch circuit. Since the fault is still present, the midpoint recloser trips and locks open. However, electrical power is still available for the customers between the midpoint recloser and the tie recloser. Thus the average duration of electrical power loss and the amount of affected customers are minimized.
However, since the fault is still present when tie recloser connects the branch circuits, the feeder recloser that protects the second or unaffected branch circuit will also detect the fault. The detection of the fault may cause the feeder recloser on the non-faulted branch circuit to trip once on a fast trip. Thus, the customers on the unaffected branch circuit will experience one momentary interruption in electrical power during a short period of time after the tie recloser connects the branch circuits and before the midpoint recloser near the fault trips. The closing of the tie recloser into a fault also creates undesirable power quality issues in the branch circuits. When the tie reclosure attempts to close into the faulted circuit, the voltage on the faulted phase sags and the unaffected phases may experience a voltage rise.
Thus, while existing electrical distribution systems are suitable for its intended purpose, there remains a need for improvements; particularly regarding the minimization or elimination of momentary electrical power interruptions and the improvement of power quality on autoloop configured branch circuits when a tie recloser is closed.